Keen bass follows the intro to Summer Love (3)
I believe the answer is:
sob
'keen' is the definition.
(thesaurus)
'bass follows the intro to summer love' is the wordplay.
'bass' becomes 'b' (abbreviation in music).
'follows' says to put letters next to each other.
'the intro to' indicates taking the first letters (I've seen 'intros to' mean this).
'love' becomes 'o' (love means zero in tennis).
The first letter of 'summer' is 's'.
's'+'o'='so'
'b' put after 'so' is 'SOB'.
(Other definitions for sob that I've seen before include "Cry noisily" , "Wep convulsively" , "Blubber" , "Weep noisily" , "Cry loudly" .)